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COPD is a lung disease that makes breathing difficult. It is a combination of chronic bronchitis and emphysema. Prolonged smoking is the most common cause of COPD; less common causes include exposure to chemical fumes, dust and pollution. The disease gets worse over time, and the damage it causes is irreversible, states WebMD.

There are four stages of COPD. Stage 1 is mild COPD with only a small amount of airflow limitation. In this stage there is little abnormality in the lungs, and the patient is often unaware that the condition exists. Stage 2 is moderate COPD with progressing airflow limitation. In this stage, patients begin to notice chronic symptoms, such as shortness of breath during exertion and chronic coughing. This is the stage where patients generally seek medical help for symptoms. Stage 3 is severe COPD with progressively worse airflow limitation. In this stage the patient’s quality of life is impacted due to increased shortness of breath and a reduction in the ability to exercise. Stage 4 is very severe COPD with significant airflow limitation, exacerbated by conditions that make the condition life-threatening, explains Healthline